Pre-packed cartridges for SPE operate on the principle of liquid–solid extraction and may be used in one of two modes:
a) the interfering matrix elements of a sample are retained on the cartridge while the components of interest are eluted;
b) the components of interest are retained while the interfering matrix elements are eluted. In the latter case, a concentration effect can be achieved.
The required compounds are then eluted from the cartridge by changing the solvent.